1. Field of the Invention
The present invention relates to the field of class D amplifiers.
2. Prior Art
Class D (audio) amplifiers for delivering between 10 mW and 10 W of power are generally avoided in high volume consumer applications because of the need for a bulky external filter to remove high frequency ripple and maximize efficiency. This passive filter will consist of at least 2 high current inductors and can easily double the cost and quadruple the printed circuit board space required to implement the class D amplifier compared with an equivalent linear class A or class AB design.
Disclosed in U.S. Pat. No. 5,077,539 is a switching amplifier system which in one embodiment includes switching circuitry responsive to a tri-state command signal for providing a power output from a power supply which has amplitude levels, time durations, and polarities corresponding to the tri-state command signal for supply to a load via a suitable filter. A further embodiment includes a linear amplifier, the switching circuitry responding to a fourth state command signal for actuating the linear amplifier to respond directly to an input signal when the amplitude of the input signal is below a selected level.
Disclosed in U.S. Pat. No. 6,211,728 is a Class-D switching amplifier having a ternary modulation scheme implemented in an H-bridge configuration. The switching amplifier has four states of operation, and achieves increased efficiency and reduced cost by delivering current to the load only when needed, and once delivered, maintaining the current. The Class-D switching amplifier is said to eliminate the need for post amplifier filters.